Steels are popular engineering materials for industrial use. They are usually alloyed with the other elements (Me: Cr, Mo, V, W, Nb, ... and N) in order to improve the mechanical and corrosion resistance properties.

The modern high-resistant steels (P91, P92, E911, VM12) [1] reached the high strength up to 100MPa at 575-650°C. It is no rare case that these steels have to be weld joined with the other steels, which have different compositions (T/P22, T/P23, T/P24, …). In the case of dissimilar weld joints [2], the mechanical properties usually take a turn for the worse because the phase microstructure changes caused by long-time diffusion during exploitation at high temperature [3]. The most important diffusing elements are the carbon and the nitrogen for the steels. The diffusion flux of these light elements is in close relation to their thermodynamic activities.

The heat-resistant steels, filler alloys, and welding electrodes can be thermodynamically treated as Fe-Me-N-C based closed systems. Phase diagrams and temperature dependent carbon and nitrogen activities of the steels can be calculated using CALPHAD approach [4].

In this contribution, the CALPHAD method is applied. Examples of phase diagrams, temperature dependences of the carbon and nitrogen activities are calculated, plotted, and discussed.

Acknovledgement: MSM/00216/22410 and GD 106/05/H008.

[1] B. Vandenberghe, et. all.: Rev. Met. Paris, N°3 (2005), 263-268.

[2] HKDH. Bhadeshia: Welding J. 83/9 (2004), 237S-243S.

[3] M. Sireesha, S.K. Albert, S. Sundaresan: Journal of Materials Engineering and Performance, 10/3 (2001), 320-330.

[4] N. Saunders, A. P. Miodovnik: 'CALPHAD (Calculation of Phase Diagram), Pergamon Materials Series, Vol. 1, Elsevier Science, Amsterodam (1998).

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